This paper presents the first results of a system being developed to follow the trajectories of motor vehicles in planar curves from a sequence of digital images in order to help civil engineers design safer roads. The method is based on mathemetical morphology that allows a segmentation of the image in such a way that the contour of a vehicle can be completely detected. The eight steps in the algorithm are described. Its use in road traffic flow monitoring is outlined and the images produced during the eight steps presented. For the covering abstract see IRRD 880251.